The present invention relates generally to coulometric titration, and more particularly to an apparatus and method for analyzing the water content in chemicals which have a relatively high vapor pressure level or which boil below atmospheric temperature, such as methyl chloride.
During the production of certain chemicals, it would be beneficial to determine the amount of particular trace constituents, such as the content of water in methyl chloride. However, providing an accurate and reproducible method of analysis is often difficult and quite operator dependent. This is particularly true with respect to the analysis of water in methyl chloride.
As is well known, methyl chloride will generate approximately 75 pounds of pressure under atmospheric or ambient temperature conditions. In other words, if a cylinder containing liquid methyl chloride is opened to the atmosphere in ambient laboratory conditions (e.g., 70.degree.-80.degree. f) it will supercool itself down to -24.5 degrees centigrade. Thus, methyl chloride has a boiling point of -24.5 degrees centigrade.
In general, the water content of methyl chloride is usually analyzed in the industry by Karl Fischer titration. This process typically involves bringing a cylinder containing a supply of methyl chloride into an analytical laboratory, weighing the cylinder, and dispersing a small sample into a titration vessel. As the cylinder is cracked open, the methyl chloride changes from a liquid into a gas as it encounters atmospheric conditions. Specifically, the methyl chloride is bubbled into the solvent contained in the titration vessel, and hopefully all of the water in the methyl chloride will be trapped in this solvent. The sample of methyl chloride or titrand is then subjected to conventional titration analysis.
In this regard, Karl Fischer's method depends on the use of a special reagent, consisting of iodine, sulphur dioxide, and pyridine. In the presence of water, the SO2 is oxidized by the iodine to SO3 from which sulphuric acid is formed with water. EQU SO2+I2+2 H2O.fwdarw.2 HI+H2SO4
The acid components (hydriodic and sulphuric acids) are taken up by the pyridine. When all the water has been consumed in this manner, free iodine remains in the solution, giving rise to the changes in current or voltage. This end point voltage is determined by a two platinum electrode and is referred to as dead-stop or biamperometric method.
With coulometric titration, the process is similar in principal, except that the titration vessel includes an electrode assembly for generating its own pyridine to neutralize the water. There are two basic advantages of coulometric titration when compared to standard Karl Fischer titration. Firstly, with coulometric titration, there is no need to repeatedly calibrate the reagent during the process. Whereas, with Karl Fischer titration, reagent is continuously added, so that there is a possibility that moisture will contaminate the reagent. Additionally, coulometric titration typically requires a much smaller amount of the sample to work with, and therefore, the analytical process is considerably faster when compared with Karl Fischer titration.
Nevertheless, even with coulometric titration, it is difficult to obtain accurate and repeatable results when analyzing the water content in methyl chloride. This is due in large part to the fact that methyl chloride boils at such a low temperature, as it has relatively high vapor pressure. For example, when the methyl chloride cylinder is cracked open, it is possible to transfer too much methyl chloride, and therefore not all of the water will be trapped in the solvent. Additionally, as the methyl chloride enters the gas phase, it will be chilling the tube downstream of the valve being cracked open, and it is possible for water to be added to the system due to this refrigeration effect. Furthermore, methyl chloride is hydroscopic. Accordingly, as a dry chemical, it will draw in moisture to itself, and this has made it almost impossible to achieve good results with commercial titration equipment for methyl chloride samples having a water content below 10 ppm.
Therefore, it is a principal objective of the present invention to provide a titration method and system for analyzing the water content in chemicals such as methyl chloride which will achieve reproducible and accurate results.
It is another objective of the present invention to achieve a method and system of titration analysis which is not technique or operator dependent in terms of obtaining accurate and reproducible readings.
It is a further objective of the present invention to provide a method and system of titration analysis which can achieve relatively quick results and be capable of automation for process control for chemical production.
It is an additional objective of the present invention to provide a method and system of titration which is capable of determining the water content in hydroscopic chemicals down below 10 ppm.
It is yet another objective of the present invention to provide a method and system of titration analysis which is capable of utilizing existing titration vessels and coulometric analyzers.